The present invention relates generally to a method and apparatus for calendering a sheet material web, and in particular, to a method and apparatus for calendering a sheet material web while being carried by one or more carrier fabrics.
It is well known in the field of paper making, and particularly in the field of manufacturing tissue products such as facial tissues, bath tissues, paper towels and the like, to provide for a continuous running sheet material web to traverse an xe2x80x9copen drawxe2x80x9d before being wound into rolls. The area of open draw, where the dried sheet web is momentarily unsupported before being wound, can provide one location to calender the web, i.e., press the web to reduce the caliper or thickness thereof.
As described in U.S. Pat. No. 5,591,309, issued Jan. 7, 1997 to Rugowski et al., and U.S. Pat. No. 5,593,545, issued Jan. 14, 1997 to Rugowski et al., both assigned to Kimberly-Clark Corporation, the same assignee as the present application, open draws are a frequent source of sheet breaks and associated production delays. As a result, tissue sheets often are designed to have high machine direction strengths in order to remain intact as they are pulled through the open draw. However, high machine direction strengths can adversely affect the quality of the web in terms of its desired softness. Therefore, as explained in U.S. Pat. Nos. 5,591,309 and 5,593,545, the elimination of open draws in the tissue manufacturing process can result in a sheet material being made more efficiently at less cost and with more desirable properties.
As a result of eliminating the open draw, the sheet material web istypically wound onto a roll and thereafter subjected to a calendering operation in a subsequent converting or finishing process. Often, the sheet material web, such as that made by the uncreped through-air-dried (UCTAD) process described in U.S. Pat. Nos. 5,591,309 and 5,593,545, will have a relatively high bulk, with the attendant benefits of increased absorbency and improved fiber utilization. In order to achieve certain operating efficiencies, however, the high-bulk sheet material web typically must be wound onto relatively large diameter rolls. Without adequate yardage deposited on each roll, the time increment-between the changing of the windup rolls can sometimes be too short from a logistic and resource management standpoint.
However, large diameter rolls, comprised of relatively loosely wound high bulk basesheets that may be susceptible to shifting between layers, may require special handling at the windup reel, during transportation from the reel to the converting or finishing stations, and/or during the converting and finishing process. For example, larger spaces may be required for interim storage, passageways between lines may need to be expanded and additional personnel and lift trucks may be needed for transportation. Moreover, line speeds at the converting and finishing lines may need to be reduced to minimize the risks typically associated with unwinding loosely wound rolls having low interlayer pressures.
Briefly stated, in one aspect, the invention is directed to an apparatus for calendering a sheet material web while being carried on or between one or more carrier fabrics. In a preferred embodiment, the apparatus includes a calender section having first and second calender rolls forming a nip therebetween. At least one carrier fabric is disposed between the first and second calender rolls.
In one exemplary embodiment, at least one carrier fabric carries the sheet material web from a drying section to a winding section. The first and second calender rolls apply a load to the carrier fabric and sheet material web as they pass through the nip between the calender rolls so as to reduce the caliper, or thickness, of the sheet material web.
In a preferred embodiment, the calender section includes a first and second carrier fabric that sandwich, or support, the sheet material web therebetween. The first and second calender rolls apply a load to the first and second carrier fabrics across the width thereof so as to calender, or press, the web supported therebetween so as to thereby reduce the caliper of the web. In a preferred embodiment, the first carrier fabric is substantially air impermeable, while the second carrier fabric is substantially air permeable.
In one embodiment, the first carrier fabric includes a reinforcing structure encapsulated in a polymer matrix. Preferably, the polymer matrix is resilient, so as to provide benefits similar to those achieved through the use of soft nip calendering. In such an embodiment, the-first and second calender rolls are preferably made of steel or like material to form a hard nip therebetween.
Alternatively, when using one or more non-resilient carrier fabrics, one or both of the first and second calender rolls can include, or be covered with, a resilient material that contacts one or more of the carrier fabrics so as to form a soft nip between the first and second calender rolls.
In another aspect of the invention, a method is provided for calendering the sheet material web as it is carried by the carrier fabric. The method includes carrying the sheet material web on one or more carrier fabrics through the calender nip formed by the first and second calender rolls and applying a load to the carrier fabric, and sheet material carried thereby, so as to reduce the thickness of the sheet material web.
In yet another aspect of the invention, the calendering apparatus, having a first and second calender roll with at least one carrier fabric disposed therebetween, can be provided on a converting or finishing line to calender or press a sheet material web so as to reduce the thickness of the web as the web is further slit, embossed, crimped or otherwise processed on the converting or finishing line.
The present invention provides significant advantages over other calendering apparatuses and processes. For example, the apparatus permits calendering of the sheet material web while it is being carried by one or more carrier fabrics, and more specifically, in one aspect, while it is being carried on one or more carrier fabrics from the drying section to the winding section. In this way, the open draw of the forming process can be eliminated so as to reduce waste and costs, but without having to calender the sheet material web the entire desired amount at a separate converting or finishing station. The resultant reduction in thickness of the sheet material web permits the manufacturer to make smaller diameter rolls having sufficient yardage for manufacturing efficiencies, and with more uniform interlayer pressures. In this way, the handling and manufacturing problems that can be encountered with large, loosely wound rolls are avoided.
The present invention, together with further objects and advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.